Photodetector and method of forming the photodetector on stacked trench isolation regions

ABSTRACT

Disclosed are structures and methods of forming the structures so as to have a photodetector isolated from a substrate by stacked trench isolation regions. In one structure, a first trench isolation region is in and at the top surface of a substrate and a second trench isolation region is in the substrate below the first. A photodetector is on the substrate aligned above the first and second trench isolation regions. In another structure, a semiconductor layer is on an insulator layer and laterally surrounded by a first trench isolation region. A second trench isolation region is in and at the top surface of a substrate below the insulator layer and first trench isolation region. A photodetector is on the semiconductor layer and extends laterally onto the first trench isolation region. The stacked trench isolation regions provide sufficient isolation below the photodetector to allow for direct coupling with an off-chip optical fiber.

BACKGROUND

The structures and methods disclosed herein relate to photodetectorsand, more particularly, to a semiconductor structure comprising aphotodetector on stacked trench isolation regions that prevent opticalsignal loss through a semiconductor substrate below and a method offorming the semiconductor structure.

Generally, in optoelectronics and, particularly, in optoelectronicintegrated circuits, on-chip photodetectors (also referred to herein asphotosensors or optical receivers) capture optical signals from otheron-chip optical devices, such as optical waveguides, and convert theoptical signals into electronic signals. In such optoelectronicintegrated circuits, oftentimes silicon is used as the core material foroptical waveguides because it is transparent to optical signals in theinfrared wavelength bands and germanium is used for photodetectorsbecause it absorbs optical signals in those same infrared wavelengthbands. While photodetectors are useful in capturing optical signals fromon-chip optical devices, they typically are not used to capture opticalsignals directly from off-chip optical devices, such as optical fibers,because of optical signal loss into the semiconductor substrate at theedge of a chip where direct coupling with the off-chip optical wouldoccur. This is the case when a photodetector is formed on a bulksemiconductor wafer as well as when a photodetector is formed on arelatively thin insulator layer of a semiconductor-on-insulator (SOI)wafer.

SUMMARY

In view of the foregoing, disclosed herein are semiconductor structuresand methods of forming the semiconductor structures. The semiconductorstructures each have a photodetector that is optically and electricallyisolated from a semiconductor substrate below by stacked trenchisolation regions. Specifically, one semiconductor structure cancomprise a first trench isolation region in and at the top surface of abulk semiconductor substrate and a second trench isolation region in thesubstrate below the first trench isolation region. A photodetector canbe on the top surface of the semiconductor substrate aligned above thefirst and second trench isolation regions. Another semiconductorstructure can comprise a semiconductor layer on an insulator layer andlaterally surrounded by a first trench isolation region. Additionally, asecond trench isolation region can be in a semiconductor substrate belowthe first trench isolation region and insulator layer. A photodetectorcan be on the semiconductor layer and can extend laterally onto thefirst trench isolation region. In each of these semiconductorstructures, the first and second trench isolation regions (i.e., thestacked trench isolations) can provide sufficient isolation below thephotodetector to allow for direct coupling with an off-chip opticaldevice (e.g., optical fiber) with minimal optical signal loss throughsemiconductor substrate.

More particularly, disclosed herein is a semiconductor structure formedon a bulk semiconductor wafer and comprising a photodetector on stackedtrench isolation regions. Specifically, this semiconductor structure cancomprise a semiconductor substrate, having a top surface, and a firsttrench isolation region in the semiconductor substrate at the topsurface. The first trench isolation region can have a first opening thatextends vertically there through. The semiconductor structure canfurther comprise a photodetector above the first opening and extendinglaterally onto the first trench isolation region. The semiconductorstructure can further comprise a second trench isolation region in thesemiconductor substrate aligned below the photodetector and first trenchisolation region. It should be noted that the first opening can befilled with an isolation material. Alternatively, the first opening canhave an upper portion immediately adjacent to the photodetector andfilled with a semiconductor material and a lower portion immediatelyadjacent to the second trench isolation region and filled with anisolation material.

Also disclosed herein is semiconductor structure formed on asemiconductor-on-insulator (SOI) wafer and comprising a photodetectorabove stacked trench isolation regions. Specifically, this semiconductorstructure can comprise a semiconductor substrate and an insulator layeron the top surface of the semiconductor substrate. This semiconductorstructure can further comprise both a first trench isolation region anda semiconductor layer on the insulator layer. The first trench isolationregion can have first opening extending vertically there through and thesemiconductor layer can be positioned within the first opening such thatthe first trench isolation region laterally surrounds the semiconductorlayer. The semiconductor structure can further comprise a photodetectoron the semiconductor layer (i.e., above the first opening) and furtherextending laterally onto the first trench isolation region. Thesemiconductor structure can further comprise a second trench isolationregion in and at the top surface of the semiconductor substrate suchthat it is immediately adjacent to the insulator layer and furtheraligned below the photodetector and first trench isolation region.

In each of the above-described semiconductor structures, thephotodetector can have a first end and a second end opposite the firstend. An antireflective spacer can be positioned laterally immediatelyadjacent to the first end of the photodetector. An optical fiber can bepositioned in end-to-end alignment with the first end of thephotodetector. Specifically, an edge of the semiconductor substrate canextend laterally beyond the first end of the photodetector and furtherbeyond the antireflective spacer. The optical fiber can be positioned onthis edge such that the antireflective spacer is positioned laterallybetween the optical fiber and the first end of the photodetector. Suchan optical fiber can transmit optical signals to the photodetector and,during transmission of these optical signals, the isolation materialthat is below the photodetector (i.e., the stacked trench isolationregions, including the first trench isolation region and the secondtrench isolation region, as well as the insulator layer, if applicable)can prevent loss of the optical signals into the semiconductorsubstrate.

Also disclosed herein is a method of forming, on a bulk semiconductorwafer, a semiconductor structure comprising a photodetector on stackedtrench isolation regions. That is, the method can comprise providing abulk semiconductor wafer comprising a semiconductor substrate having atop surface.

The method can further comprise forming a first trench isolation regionin the semiconductor substrate at the top surface. Specifically, themethod can comprise forming a first trench at the top surface of thesemiconductor substrate and filling the first trench with a firstisolation material in order to form a first trench isolation region. Itshould be noted that the first trench should be formed (e.g., patternedand etched) such that the resulting first trench isolation region has afirst opening and such that a portion of the semiconductor substrate iswithin the first opening laterally surrounded by the first trenchisolation region.

This method can further comprise forming a photodetector on the portionof the semiconductor substrate within the first opening. After formingthe photodetector, a dielectric layer can be formed such that it coversthe photodetector and further extends laterally onto the first trenchisolation region.

Then, a second trench isolation region can be formed in thesemiconductor substrate aligned below the photodetector and the firsttrench isolation region. Specifically, a plurality of second openingscan be formed such that they extend vertically through the dielectriclayer and the first trench isolation region into the semiconductorsubstrate and further such that they are positioned adjacent to multiplesides of the photodetector. Once the second openings are formed, exposedsurfaces of the semiconductor substrate in the second openings can beetched in order to form a second trench in the semiconductor substratebelow the first trench isolation region. This second trench can then befilled with a second isolation material so as to form the second trenchisolation region.

It should be noted that, during the etch process used to form the secondtrench, the portion of the semiconductor substrate within the firstopening in the first trench isolation can be etched from below. Thus,the process of filling the second trench with the second isolationmaterial can also result in the first opening being at least partiallyfilled with the second isolation material. That is, if all of thesemiconductor material within the first opening is etched out, the firstopening may subsequently be filled with the second isolation material.Alternatively, if only the semiconductor material within the lowerportion of the first opening is etched out, only the lower portion ofthe first opening will be filled with the second isolation material andthe upper portion will remain filled with semiconductor material.

Also disclosed herein is a method of forming, on asemiconductor-on-insulator (SOI) wafer, a semiconductor structure with aphotodetector on stacked trench isolation regions. Specifically, thismethod can comprise providing semiconductor-on-insulator (SOI) wafercomprising a semiconductor substrate, an insulator layer on the topsurface of the semiconductor substrate, and a semiconductor layer on theinsulator layer.

The method can further comprise forming a first trench isolation regionin the semiconductor layer. Specifically, the method can compriseforming a first trench in the semiconductor layer and filling the firsttrench with a first isolation material in order to form a first trenchisolation region. It should be noted that the first trench should beformed (e.g., patterned and etched) such that the resulting first trenchisolation region has a first opening and such that a portion of thesemiconductor layer is within the first opening laterally surrounded bythe first trench isolation region.

This method can further comprise forming a photodetector on the portionof the semiconductor layer within the first opening. After forming thephotodetector, a dielectric layer can be formed such that it covers thephotodetector and further extends laterally onto the first trenchisolation region.

Then, a second trench isolation region can be formed in thesemiconductor substrate below the insulator layer such that it isaligned below the photodetector and the first trench isolation region.Specifically, a plurality of second openings can be formed such thatthey extend vertically through the dielectric layer, the first trenchisolation region and the insulator layer into the semiconductorsubstrate and further such that they are positioned adjacent to multiplesides of the photodetector. Once the second openings are formed, exposedsurfaces of the semiconductor substrate in the second openings can beetched in order to form a second trench in the semiconductor substratebelow the insulator layer and, specifically, aligned below thephotodetector and the first trench isolation region. This second trenchcan then be filled with a second isolation material so as to form thesecond trench isolation region.

In each of the above-described methods, the photodetector can have afirst end and a second end opposite the first end. An antireflectivespacer can be formed so that it is positioned laterally immediatelyadjacent to the first end of the photodetector. An optical fiber canalso be positioned such that it is in end-to-end alignment with thefirst end of the photodetector. Specifically, an edge of thesemiconductor substrate can extend laterally beyond the first end of thephotodetector and further beyond the antireflective spacer. The opticalfiber can be positioned on this edge such that the antireflective spaceris positioned laterally between the optical fiber and the first end ofthe photodetector. Such an optical fiber can transmit optical signals tothe photodetector and, during transmission of these optical signals, theisolation material that is below the photodetector (i.e., the stackedtrench isolation regions, including the first trench isolation regionand the second trench isolation region, as well as the insulator layer,if applicable) can prevent loss of the optical signals into thesemiconductor substrate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, which are notnecessarily drawn to scale and in which:

FIG. 1A is a cross-section illustration of a semiconductor structure,which is formed on a bulk semiconductor substrate and which has aphotodetector above stacked trench isolation regions;

FIG. 1B is another cross-section illustration of the semiconductorstructure of FIG. 1A;

FIG. 1C is yet another cross-section illustration of the semiconductorstructure of FIG. 1A;

FIG. 2A is a cross-section illustration of a semiconductor structure,which is formed on a bulk semiconductor substrate and which has aphotodetector above stacked trench isolation regions;

FIG. 2B is another cross-section illustration of the semiconductorstructure of FIG. 2A;

FIG. 2C is yet another cross-section illustration of the semiconductorstructure of FIG. 2A;

FIG. 3 is a cross-section illustration of an alternative embodiment ofthe semiconductor structure of FIGS. 1A-1C;

FIG. 4 is a cross-section illustration of an alternative embodiment ofthe semiconductor structure of FIG. 2A-2C;

FIG. 5 is a flow diagram illustrating a method of forming thesemiconductor structure of FIGS. 1A-1C (or FIG. 3);

FIG. 6 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 5;

FIG. 7 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 5;

FIG. 8 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 5;

FIG. 9A is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 5;

FIG. 9B is a top view diagram illustrating the partially completedsemiconductor structure of FIG. 9A;

FIG. 10 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 5;

FIG. 11 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 5;

FIG. 12 is a flow diagram illustrating a method of forming thesemiconductor structure of FIGS. 2A-2C (or FIG. 4);

FIG. 13 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 12;

FIG. 14 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 12;

FIG. 15 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 12;

FIG. 16A is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 12;

FIG. 16B is a top view diagram illustrating the partially completedsemiconductor structure of FIG. 16A;

FIG. 17 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 12; and,

FIG. 18 is a cross-section illustration of a partially completedsemiconductor structure formed according to the method of FIG. 12.

DETAILED DESCRIPTION

As mentioned above, in optoelectronics and, particularly, inoptoelectronic integrated circuits, on-chip photodetectors (alsoreferred to herein as photosensors or optical receivers) capture opticalsignals from other on-chip optical devices, such as optical waveguides,and convert the optical signals into electronic signals. In suchoptoelectronic integrated circuits, oftentimes silicon is used as thecore material for optical waveguides because it is transparent tooptical signals in the infrared wavelength bands and germanium is usedfor photodetectors because it absorbs optical signals in those sameinfrared wavelength bands. While photodetectors are useful in capturingoptical signals from on-chip optical devices, they typically are notused to capture optical signals directly from off-chip optical devices,such as optical fibers, because of optical signal loss into thesemiconductor substrate at the edge of a chip where direct coupling withthe off-chip optical would occur. This is the case when a photodetectoris formed on a bulk semiconductor wafer as well as when a photodetectoris formed on a relatively thin insulator layer of asemiconductor-on-insulator (SOI) wafer.

In view of the foregoing, disclosed herein are semiconductor structuresand methods of forming the semiconductor structures. The semiconductorstructures each have a photodetector that is optically and electricallyisolated from a semiconductor substrate below by stacked trenchisolation regions. Specifically, one semiconductor structure cancomprise a first trench isolation region in and at the top surface of abulk semiconductor substrate and a second trench isolation region in thesubstrate below the first trench isolation region. A photodetector canbe on the top surface of the semiconductor substrate aligned above thefirst and second trench isolation regions. Another semiconductorstructure can comprise a semiconductor layer on an insulator layer andlaterally surrounded by a first trench isolation region. Additionally, asecond trench isolation region can be in a semiconductor substrate belowthe first trench isolation region and insulator layer. A photodetectorcan be on the semiconductor layer and can extend laterally onto thefirst trench isolation region. In each of these semiconductorstructures, the first and second trench isolation regions (i.e., thestacked trench isolations) can provide sufficient isolation below thephotodetector to allow for direct coupling with an off-chip opticaldevice (e.g., optical fiber) with minimal optical signal loss throughsemiconductor substrate.

More particularly, FIGS. 1A-1C in combination illustrate a semiconductorstructure 100 formed on a bulk semiconductor wafer and comprising aphotodetector 130 on stacked trench isolation regions (i.e., see firsttrench isolation region 110 above the second trench isolation region120). FIG. 1A is a cross-section illustration of the semiconductorstructure 100 through a vertical plane, which cuts across the length ofthe structure. FIG. 1B is another cross-section illustration through adifferent vertical plane A-A′, which, as shown in FIG. 1A, cuts acrossthe width of the structure. FIG. 1C is yet another cross-sectionillustration of the semiconductor structure 100 through a horizontalplane B-B′, which, as shown in FIG. 1A, cuts across the semiconductorsubstrate near the top surface.

Specifically, referring to FIGS. 1A-1C, the semiconductor structure 100can comprise a semiconductor substrate 101 having a top surface 105.This semiconductor substrate 101 can comprise, for example, a siliconsubstrate or other suitable semiconductor substrate.

The semiconductor structure 100 can further comprise a first trenchisolation region 110 in the semiconductor substrate 101 at the topsurface 105. The first trench isolation region 110 can comprise a firsttrench 111 and the first trench 111 can be filled with one or more firstisolation materials 112 (e.g., silicon dioxide, silicon nitride, siliconoxynitride, and/or any other suitable isolation material). As discussedin greater detail below with regard to the method of forming thissemiconductor structure 100, the first trench 111 can be patterned andetched such that the resulting first trench isolation region 110 has afirst opening 115 that extends vertically there through. This firstopening 115 can, for example, be an essentially rectangular-shaped firstopening.

The semiconductor structure 100 can further comprise a photodetector 130aligned above the first opening 115 in the first trench isolation region110. This photodetector 130 can, for example, have the same shape as thefirst opening 115 (e.g., can have an essentially rectangular shape) andcan be slightly larger than the first opening 115 such that it extendslaterally onto the first trench isolation region 110. The photodetector130 can have a first end 133, a second end 134 opposite the first end133, and opposing sides 135. The photodetector 130 can comprise alight-absorbing layer 131. This light-absorbing layer 131 can absorblight (i.e., optical signals or photons) within predetermined wavelengthbands. For example, this light-absorbing layer 131 can comprise agermanium layer (e.g., an epitaxial germanium layer) that absorbs light(i.e., optical signals or photons) in the infrared wavelength bands(i.e., wavelengths (λ) in the range of approximately 1.2μ to 1.7μ). Thegermanium layer can be doped so as to have N-type conductivity, P-typeconductivity, or alternating regions of N-type conductivity and P-typeconductivity (e.g., to form a PN junction diode or a multitude of PNjunctions within the germanium layer). The doping concentrations canrange from 1e17 atoms/cm³ to 1e21 atoms/cm³ with preferableconcentrations between 1e19 atoms/cm³ to 1e20 atoms/cm³. The dopingprofiles within the germanium layer can, for example, be constructedsuch that the peak concentration is located at the half-height of thegermanium layer. Furthermore, the doping profiles within the germaniumlayer can be optimized to reduce dark current of the photodetector,while providing for high responsivity and high operation speed. Inanother example, the light-absorbing layer 131 can comprise agermanium-tin layer or any other suitable light-absorbing layer. Suchlight-absorbing layers can similar be doped so as to have N-typeconductivity, P-type conductivity, or alternating regions of N-typeconductivity and P-type conductivity. Optionally, this photodetector 130can further comprise a dielectric cap 136 above the light-absorbinglayer 131 and/or a buffer layer 132 stacked between the light-absorbinglayer 131 and the first trench isolation region 110 at the top surface105 of the semiconductor substrate 101. The dielectric cap 136 cancomprise, for example, a silicon nitride cap or other suitabledielectric cap. The buffer layer 132 can comprise, for example, asilicon germanium layer (e.g., an epitaxially deposited silicongermanium layer) that facilitates subsequent epitaxial deposition of agermanium light-absorbing layer.

The semiconductor structure 100 can further comprise a dielectric layer141 covering the photodetector 130 and further extending laterally ontothe first trench isolation region 110 beyond the photodetector 130. Thisdielectric layer 141 can, for example, comprise a silicon nitride layer,a silicon oxynitride layer or any other suitable semiconductor layer. Aplurality of second openings 116 can be positioned adjacent to multiplesides of photodetector 130 and, particularly, adjacent to at least theopposing sides 135 and the first end 133 of the photodetector 130. Thesesecond openings 116 can extend vertically through dielectric layer 141and the first trench isolation region 110.

The semiconductor structure 100 can further comprise a second trenchisolation region 120 in the semiconductor substrate 101 aligned belowthe photodetector 130 and the first trench isolation region 110. Asdiscussed in greater detail below with regard to the method of formingthis semiconductor structure 100, this second trench isolation region120 can comprise a second trench 121 that is formed by performing anetch process that expands the lower sections of the second openings 116within the semiconductor substrate 101 immediately below the firsttrench isolation region 110 until those lower sections are merged. As aresult, the second trench 121 is aligned below the photodetector 130 andthe first trench isolation region 110. It should be noted that, due tothe etch process used when forming the second trench 121, the sidewalls126 of the second trench 121 may be angled relative to the bottomsurface 125 of the second trench 121 (e.g., as opposed toperpendicular). In any case, this second trench 121 can be filled withone or more second isolation materials 122 (e.g., silicon dioxide,silicon nitride, silicon oxynitride and/or any other suitable isolationmaterial). The second isolation material(s) can be the same as ordifferent from the first isolation material(s) 112. It should be notedthat, as a result of the deposition process used to fill the secondtrench 121 with the second isolation material(s) 122, voids or airgaps(not shown) might be present within the second trench isolation region120.

The second openings 116 can similarly be filled with the secondisolation material(s) 122. Optionally, the first opening 115, which asmentioned above extends vertically through the first trench isolationregion 110 and, thus, extends vertically between the photodetector 130and the second trench isolation region 120, can also be filled with thesecond isolation material(s) 122. Alternatively, this first opening 115can be only partially filled with the second isolation material(s) 122.That is, this first opening 115 can have an upper portion, which isimmediately adjacent to the photodetector 130 and which is filled with asemiconductor material (i.e., a portion 104 of the semiconductorsubstrate 101 at the top surface 105) and can also have a lower portion,which is immediately adjacent to the second trench isolation region 120and which is filled with the second isolation material(s) 122.

FIGS. 2A-2C in combination illustrate another semiconductor structure200. This semiconductor structure 200 is formed on asemiconductor-on-insulator (SOI) wafer and comprises a photodetector 230on stacked trench isolation regions (i.e., see first trench isolationregion 210 above the insulator layer 203 and second trench isolationregion 220). FIG. 2A is a cross-section illustration of thesemiconductor structure 200 through a vertical plane, which cuts acrossthe length of the structure. FIG. 2B is another cross-sectionillustration through a different vertical plane A-A′, which, as shown inFIG. 2A, cuts across the width of the structure. FIG. 2C is yet anothercross-section illustration of the semiconductor structure 200 through ahorizontal plane B-B′, which, as shown in FIG. 2A, cuts across thesemiconductor substrate near the top surface.

Specifically, referring to FIGS. 2A-2C, the semiconductor structure 200can comprise a semiconductor substrate 201, having a top surface 205 andan insulator layer 203 on the top surface of the semiconductor substrate201 and a semiconductor layer 204 on the insulator layer 203. Thesemiconductor substrate 201 can comprise, for example, a siliconsubstrate or other suitable semiconductor substrate. The insulator layer203 can comprise, for example, a silicon dioxide layer or other suitableinsulator layer. The semiconductor layer 204 can comprise, for example,a silicon layer, a silicon germanium layer or other suitablesemiconductor layer 204.

The semiconductor structure 200 can further comprise a first trenchisolation region 210 on the insulator layer 203. The first trenchisolation region 210 can comprise a first trench 211 and the firsttrench 211 can be filled with one or more first isolation materials 212(e.g., silicon dioxide, silicon nitride, silicon oxynitride, and/or anyother suitable isolation material). As discussed in greater detail belowwith regard to the method of forming this semiconductor structure 200,the first trench 211 can be patterned and etched such that the resultingfirst trench isolation region 210 has a first opening 215 that extendsvertically there through and the semiconductor layer 204 can bepositioned within the first opening 215 such that the first trenchisolation region 210 laterally surrounds and defines the shape of thesemiconductor layer 204. This first opening 215 can, for example, be anessentially rectangular-shaped first opening. Optionally, one or moredielectric columns 217 can extend vertically through the semiconductorlayer 204 within the first opening 215. As discussed in greater detailbelow with regard to the method of forming this semiconductor structure200, the dielectric columns 217 can, for example, be formed duringformation of the first trench isolation region 210 such that theycomprise the same first isolation material(s) 212.

The semiconductor structure 200 can further comprise a photodetector 230aligned above the first opening 215. This photodetector 230 can, forexample, have the same shape as the first opening 215 (e.g., can have anessentially rectangular shape) and can be slightly larger than the firstopening 215 such that it extends laterally onto the first trenchisolation region 210. The photodetector 230 can have a first end 233, asecond end 234 opposite the first end 233, and opposing sides 235. Thephotodetector 230 can comprise a light-absorbing layer 231. Thislight-absorbing layer 231 can absorb light (i.e., optical signals orphotons) within predetermined wavelength bands. For example, thislight-absorbing layer 231 can comprise a germanium layer (e.g., anepitaxial germanium layer) that absorbs light (i.e., optical signals orphotons) in the infrared wavelength bands (i.e., wavelengths (λ) in therange of approximately 1.2μ to 1.7μ). The germanium layer can be dopedso as to have N-type conductivity, P-type conductivity, or alternatingregions of N-type conductivity and P-type conductivity (e.g., to form aPN junction diode or a multitude of PN junctions within the germaniumlayer). The doping concentrations can range from 1e17 atoms/cm³ to 1e21atoms/cm³ with preferable concentrations between 1e19 atoms/cm³ to 1e20atoms/cm³. The doping profiles within the germanium layer can, forexample, be constructed such that the peak concentration is located atthe half-height of the germanium layer. Furthermore, the doping profileswithin the germanium layer can be optimized to reduce dark current ofthe photodetector, while providing for high responsivity and highoperation speed. In another example, the light-absorbing layer 231 cancomprise a germanium-tin layer or any other suitable light-absorbinglayer. Such light-absorbing layers can similar be doped so as to haveN-type conductivity, P-type conductivity, or alternating regions ofN-type conductivity and P-type conductivity. Optionally, thisphotodetector 230 can further comprise a dielectric cap 236 above thelight-absorbing layer 231 and/or a buffer layer 232 stacked between thelight-absorbing layer 231 and the semiconductor layer 204. Thedielectric cap 236 can comprise, for example, a silicon nitride cap orother suitable dielectric cap. The buffer layer 232 can comprise, forexample, a silicon germanium layer (e.g., an epitaxially depositedsilicon germanium layer) that facilitates subsequent epitaxialdeposition of a germanium light-absorbing layer.

The semiconductor structure 200 can further comprise a dielectric layer241 covering the photodetector 230 and further extending laterally ontothe first trench isolation region 110 beyond the photodetector 230. Thisdielectric layer 241 can, for example, comprise a silicon nitride layer,a silicon oxynitride layer or any other suitable semiconductor layer. Aplurality of second openings 216 can be positioned adjacent to multiplesides of the photodetector 230 and, particularly, adjacent to at leastthe opposing sides 235 and the first end 233 of the photodetector 230.These second openings 216 can extend vertically through dielectric layer241, the first trench isolation region 210 and the insulator layer 203.

The semiconductor structure 200 can further comprise a second trenchisolation region 220 in the semiconductor substrate 201 below andimmediately adjacent to the insulator layer 203 and, particularly,aligned below the photodetector 230 and the first trench isolationregion 210. As discussed in greater detail below with regard to themethod of forming this semiconductor structure 200, this second trenchisolation region 220 can comprise a second trench 221 that is formed byperforming an etch process that expands the lower sections of the secondopenings 216 within the semiconductor substrate 201 immediately belowthe insulator layer 203 until those lower sections are merged. As aresult, the second trench 221 is aligned below the photodetector 230 andthe first trench isolation region 210. It should be noted that, due tothe etch process used when forming the second trench 221, the sidewalls226 of the second trench 221 may be angled relative to the bottomsurface 225 of the second trench 221 (e.g., as opposed toperpendicular). In any case, this second trench 221 can be filled withone or more second isolation materials 222 (e.g., silicon dioxide,silicon nitride, silicon oxynitride and/or any other suitable isolationmaterial). The second isolation material(s) 222 can be the same as ordifferent from the first isolation material(s). It should be noted that,as a result of the deposition process used to fill the second trench 221with the second isolation material(s) 222, voids or airgaps (not shown)might be present within the second trench isolation region 220. Thesecond openings 216 can similarly be filled with the second isolationmaterial(s) 222.

Each of the above-described semiconductor structures 100 of FIGS. 1A-1Cand 200 of FIGS. 2A-2C can further comprise an antireflective (AR)spacer 160, 260 positioned laterally adjacent to the first end 133, 233and, particularly, positioned laterally adjacent to the lightsignal-receiving end of the photodetector 130, 230. This antireflective(AR) spacer 160, 260 can comprise, for example, titanium nitride or anyother suitable antireflective material. This antireflective (AR) spacer160, 260 can further have a quarter-wave thickness or multiple thereof.That is, the thickness of the antireflective (AR) spacer can be ¼ thewavelength of the optical signals, which are intended to be transmittedto and captured by the photodetector 130, 230.

Each of the above-described semiconductor structures 100 of FIGS. 1A-1Cand 200 of FIGS. 2A-2C can further comprise one or more additionaldielectric layers 142, 242 (e.g., interlayer dielectrics) above thedielectric layer 141, 241. The additional dielectric layer(s) 142, 242can comprise, for example, one or more layers of any of the followingdielectric materials: silicon dioxide, silicon nitride, siliconoxynitride, borophosphosilicate glass (BPSG), etc. One or more contactsand other interconnects (e.g., wires and vias) within the additionaldielectric layer(s) 142, 242 can electrically connect the photodetector130, 230 to one or more other on-chip devices.

Additionally, in each of the above-described semiconductor structures100 of FIGS. 1A-1C and 200 of FIGS. 2A-2C, the photodetector 130, 230can be optically coupled to an off-chip optical fiber 150, 250. That is,an exposed edge 190, 290 of the semiconductor substrate 101, 201 canextend laterally beyond the first end 133, 233 of the photodetector 130,230, beyond the antireflective (AR) spacer 160, 260, beyond the stackedtrench isolation regions and, if applicable, beyond the insulator layer.This exposed edge 190, 290 can have a groove (e.g., a V-groove) forreceiving an off-chip optical fiber 150, 250. An end of the off-chipoptical fiber 150, 250 can be positioned in the groove on this edge 190,290 adjacent to the antireflective (AR) spacer 160, 260 such that theoptical fiber 150, 250 is in end-to-end alignment with the first end133, 233 of the photodetector 130, 230 and, thereby such that it isoptically coupled to the photodetector 130, 230. The optical fiber 150,250 can comprise a core 151, 251 and cladding 152, 252 around this core151, 251. Both the core 151, 251 and the cladding 152, 252 can compriselight-transmissive materials; however, the core material(s) can have arefractive index that is higher than that of the cladding material(s) sothat light signals can be confined to and propagated along the core.Such an optical fiber 150, 250 can transmit optical signals to thephotodetector 130, 230 so that the photodetector 130, 230 can convertthe optical signals into electrical signals that can be transmitted toone or more other on-chip devices through the contacts and interconnectsdescribed above.

As illustrated in FIGS. 1A and 2A, in each of the semiconductorstructures 100, 200 disclosed herein, the photodetector 130, 230 canhave a height 139, 239 that is less than the diameter 159, 259 of thecore 151, 251 of the optical fiber 150, 250. However, alternatively, asillustrated in FIGS. 3 and 4, in each of the semiconductor structures100, 200, the photodetector 130, 230 can have a height 139, 239 that isapproximately equal to the diameter 159, 259 of the core 151, 251 of theoptical fiber 150, 250 for better optical coupling. In any case, duringtransmission of the optical signals from the optical fiber 150, 250 tothe photodetector 130, 230, the isolation material that is below thephotodetector 130, 230 (i.e., the stacked trench isolation regions,including the first trench isolation region 110, 210 and the secondtrench isolation region 120, 220, as well as the insulator layer 203, ifapplicable) can prevent optical signal loss into the semiconductorsubstrate 101, 201 below. Furthermore, in the semiconductor structure200 of FIGS. 2A-2C the optional dielectric columns 217 can be used tominimize optical signal loss into the semiconductor layer 204.

Referring to the flow diagram of FIG. 5, also disclosed herein is amethod of forming, on a bulk semiconductor wafer, the semiconductorstructure 100, as described in detail above, comprising a photodetector130 on stacked trench isolation regions (i.e., the first trenchisolation region 110 and the second trench isolation region 120).

Specifically, the method can comprise providing a bulk semiconductorwafer comprising a semiconductor substrate 101 having a top surface 105(502). This semiconductor substrate 101 can comprise, for example, asilicon substrate or other suitable semiconductor substrate.

The method can further comprise forming a first trench isolation region110 in the semiconductor substrate 101 at the top surface 105 (504, seeFIG. 6). To form the first trench isolation region 110, a first trench111 can be formed (e.g., lithographically patterned and etched) at thetop surface of the semiconductor substrate 101. This first trench 111can subsequently be filled with one or more first isolation materials112 (e.g., silicon dioxide, silicon nitride, silicon oxynitride, and/orany other suitable isolation material). It should be noted that thefirst trench 111 can be patterned and etched such that the resultingfirst trench isolation region 110 has a first opening 115 extendingvertically there through and such that a portion 104 of thesemiconductor substrate 101 is within the first opening 115 laterallysurrounded by the first trench isolation region 110. Thus, the shape ofthe first opening 115 of the first trench isolation region 110 definesthe shape of this portion 104 of the semiconductor substrate 101. Thisfirst opening 115 can, for example, be an essentially rectangular-shapedfirst opening.

This method can further comprise forming a photodetector 130 on the topsurface 105 aligned above the first opening 115 (506, see FIG. 7).Specifically, a light-absorbing layer 131 can be formed (e.g.,epitaxially deposited) on the top surface 105 of the semiconductorsubstrate 101 (e.g., at the portion 104 defined by the first trenchisolation region 110) such that it extends laterally over the firsttrench isolation region 110. This light-absorbing layer 131 can comprisea light absorbing material and, particularly, a material that absorbslight (i.e., optical signals or photons) within predetermined wavelengthbands. For example, this light-absorbing layer 131 can comprise agermanium layer (e.g., an epitaxial germanium layer) that absorbs light(i.e., optical signals or photons) in the infrared wavelength bands(i.e., wavelengths (λ) in the range of approximately 1.2μ, to 1.7μ). Thegermanium layer can be in-situ doped during epitaxial deposition (orsubsequently doped) so as to have N-type conductivity, P-typeconductivity, or alternating regions of N-type conductivity and P-typeconductivity (e.g., to form a PN junction diode or a multitude of PNjunctions within the germanium layer). The doping concentrations canrange from 1e17 atoms/cm³ to 1e21 atoms/cm³ with preferableconcentrations between 1e19 atoms/cm³ to 1e20 atoms/cm³. The dopingprofiles within the germanium layer can, for example, be constructedsuch that the peak concentration is located at the half-height of thegermanium layer. Furthermore, the doping profiles within the germaniumlayer can be optimized to reduce dark current of the photodetector,while providing for high responsivity and high operation speed. Inanother example, the light-absorbing layer 131 can comprise agermanium-tin layer or any other suitable light-absorbing layer. Suchlight-absorbing layers can similar be doped so as to have N-typeconductivity, P-type conductivity, or alternating regions of N-typeconductivity and P-type conductivity. Optionally, prior to formation ofthe light-absorbing layer 131, a buffer layer 132 can be formed (e.g.,epitaxially deposited) and, following formation of the light-absorbinglayer 131, a dielectric cap layer 136 can be formed (e.g., deposited).The buffer layer 132 can comprise, for example, a silicon germaniumlayer (e.g., an epitaxially deposited silicon germanium layer) thatfacilitates subsequent epitaxial deposition of a germaniumlight-absorbing layer. The dielectric cap layer 136 can comprise, forexample, a silicon nitride cap layer or other suitable dielectric caplayer. Those skilled in the art will recognize that if theabove-described optional buffer layer 132 and light-absorbing layer 131are formed by epitaxial deposition, such processes are typicallyfollowed by an anneal. In any case, the light-absorbing layer 131 and,if applicable, the buffer layer 132 and dielectric cap layer 136 cansubsequently be lithographically patterned and etched to form thephotodetector 130 such that the photodetector 130 is aligned above thefirst opening 115 and is slightly larger than the first opening 115 soas to extend laterally onto the first trench isolation region 110. Theresulting photodetector 130 can, for example, have essentially the sameshape as the first opening 115 (e.g., an essentially rectangular shape)with a first end 133, a second end 134 opposite the first end 133, andopposing sides.

It should be noted that the photodetector 130 can specifically be formedat process 506 so that its height 139 will be less than the diameter 159of a core 151 of an optical fiber 150 to which it will be coupled in theresulting semiconductor structure 100 (see FIG. 1A). Alternatively, thephotodetector 130 can specifically be formed at process 506 so that itsheight 139 will be approximately equal to the diameter 159 of a core 151of an optical fiber 150 to which it will be coupled in the resultingsemiconductor structure 100 (see FIG. 3).

In any case, after forming the photodetector 130, a dielectric layer 141can be formed (e.g., conformally deposited) such that it covers thephotodetector 130 and further extends laterally onto the first trenchisolation region 110 (508, see FIG. 8). This dielectric layer 141 cancomprise, for example, a silicon nitride layer, a silicon oxynitridelayer or any other suitable semiconductor layer.

Then, a second trench isolation region 120 can be formed in thesemiconductor substrate 101 aligned below the photodetector 130 and thefirst trench isolation region 110 (510). Specifically, a plurality ofsecond openings 116 can be formed (e.g., lithographically patterned andetched) such that they extend vertically through the dielectric layer141 and the first trench isolation region 110 into the semiconductorsubstrate 101 and further such that they are positioned adjacent tomultiple sides of the photodetector 130 and, particularly, adjacent toat least the first end 133 (i.e., the light-receiving end) and theopposing sides 135 of the photodetector 130 (see FIGS. 9A-9B). Once thesecond openings 116 are formed, exposed surfaces of the semiconductorsubstrate 101 in the lower sections of the second openings 116 can beetched until those lower sections are merged in order to form a secondtrench 121 in the semiconductor substrate 101 and this second trench 121and the second openings 116 can then be filled with one or more secondisolation material(s) 122 so as to form the second trench isolationregion 120 aligned below the photodetector 130 and the first trenchisolation region 110 (see FIG. 10). The second isolation material(s) 122can comprise, for example, silicon dioxide, silicon nitride, siliconoxynitride and/or any other suitable isolation material. The secondisolation material(s) 122 can be the same as or different from the firstisolation material(s) 112.

The specifications for the above-described etch process used to form thesecond trench 121 should specifically be chosen so that thesemiconductor material of the semiconductor substrate 101 will beselectively etched over the materials used for the dielectric layer 141and first trench isolation region 110. For example, if the semiconductorsubstrate 101 comprises silicon, if the first isolation material 112 ofthe first trench isolation region 110 comprises silicon dioxide, and ifthe dielectric layer 141 comprises silicon nitride, the etch processused to form the second trench 121 can comprise a wet chemical etchprocess, which uses an etchant, such as tetramethylammonium hydroxide(TMAH), ammonium hydroxide (NH₄OH), ethylenediamine pyrocatechol (EDP),potassium hydroxide (KOH), or any other suitable etchant capable ofetching silicon over the various dielectric and insulator materials.Those skilled in the art will recognize that alternative etchants couldbe used depending upon the chemical differences between thesemiconductor substrate 101, the dielectric layer 141 and the firstisolation material(s) of the first trench isolation region 110. Thoseskilled in the art will recognize that such wet chemical etch processestypically also exhibit etch selectivity along crystal planes (e.g.,selectivity for silicon that is significantly higher in the [100]direction than in the [111] direction). As a result, the bottom surface125 of the second trench 121 may remain essentially parallel to the topsurface 105 of the semiconductor substrate 101, but the sidewalls 126may be angled, as opposed to perpendicular, relative to the top surface105 of the semiconductor substrate 101.

Additionally, it should be noted that remaining sections of thedielectric layer 141 and first trench isolation region 110 (e.g.,between the second openings 116) as well as the adjacent substratematerial provide adequate support for the photodetector 130 duringetching of the second trench 121 and prior to filling the second trench121 with the second isolation material(s) 122. Furthermore, it should benoted that, during the above-described etch process used to form thesecond trench 121, the portion 104 of the semiconductor substrate 101within the first opening 115 in the first trench isolation region 110will eventually be exposed to the etchant used and, thereby etched frombelow so that it is either entirely removed or at least partiallyremoved. Thus, the process of filling the second trench 121 with thesecond isolation material(s) 122 can also result in the first opening115 being completely filled with the second isolation material(s) 122 orpartially filled with the second isolation material(s) (as illustratedin FIG. 10). That is, if all of the semiconductor material of theportion 104 of the semiconductor substrate 101 within the first opening115 is etched out during formation of the second trench 121, then thefirst opening 115 may subsequently be completely filled with the secondisolation material(s) 122. Alternatively, if only the semiconductormaterial within the lower portion of the first opening 115 is etchedout, then only the lower portion of the first opening 115 will be filledwith the second isolation material(s) 122 and the upper portion willremain filled with semiconductor material.

Next, an antireflective (AR) spacer 160 can be formed such that it ispositioned laterally immediately adjacent to the first end 133 and,particularly, immediately adjacent to the light signal-receiving end ofthe photodetector 130 (512, see FIG. 11). For example, a trench can belithographically patterned and etched through the dielectric layer 141and into the first trench isolation region 110 immediately adjacent tothe first end 133 of the photodetector 130. This trench can subsequentlybe filled with an antireflective (AR) material in order to form theantireflective (AR) spacer 160. The antireflective (AR) material cancomprise, for example, titanium nitride or any other suitableantireflective material. This antireflective (AR) spacer 160 should beformed at process 512 so as to have a quarter-wave thickness or multiplethereof. That is, the thickness of the antireflective (AR) spacer can be¼ the wavelength of the optical signals, which are intended to betransmitted to and captured by the photodetector 130.

Following formation of the antireflective (AR) spacer 160, back end ofthe line (BEOL) processing can be performed in order to form contactsand other interconnects (e.g., wires and vias) in one or more additionaldielectric layers 142 (i.e., interlayer dielectrics such as, silicondioxide, silicon nitride, silicon oxynitride, borophosphosilicate glass(BPSG), etc.) above the dielectric layer 141 in order to electricallyconnect the photodetector 130 to one or more other on-chip devices (514,see FIGS. 1A-1C). Additionally, an edge 190 of the semiconductorsubstrate 101 adjacent to the first end 133 of the photodetector 130 canbe prepared for receiving an off-chip optical fiber 150 so that opticalfiber 150 can be coupled to the photodetector 130 (516, see FIGS.1A-1C). As mentioned above with regard to the semiconductor structure100, an optical fiber 150 can comprise a core 151 and cladding 152around this core 151. Both the core 151 and the cladding 152 cancomprise light-transmissive materials; however, the core material(s) canhave a refractive index that is higher than that of the claddingmaterial(s) so that light signals can be confined to and propagatedalong the core. To prepare the edge 190 of the semiconductor substrate101 for receiving an optical fiber 150, this edge 190 can be exposed(e.g., using a masked etch process) such that it extends laterallybeyond that first end 133 of the photodetector 130, the antireflective(AR) spacer 160, the first trench isolation region 110 and the secondtrench isolation region 120. Then, a groove (e.g., a V-groove) forreceiving the optical fiber 150 can be formed (e.g., lithographicallypatterned and etched) on the exposed edge 190 such that it is alignedthe photodetector 130.

After the edge 190 of the semiconductor substrate 101 is prepared forreceiving an optical fiber, as described above, one end of the opticalfiber 150 can be positioned within the groove and adjacent to theantireflective (AR) spacer 160 such that it is in end-to-end alignmentwith the first end 133 of the photodetector 130 and, thereby such thatit is optically coupled to the photodetector 130 (518, see FIGS. 1A-1C).Once the optical fiber 150 is coupled to the photodetector 130 in thismanner, the optical fiber 150 can transmit optical signals to thephotodetector 130 and the photodetector 130 can convert those opticalsignals into electrical signals, which can, in turn, be transmitted toone or more other on-chip devices through the contacts and interconnectsdescribed above. During transmission of the optical signals from theoptical fiber 150 to the photodetector 130, the isolation material thatis below the photodetector 130 (e.g., in the stacked trench isolationregions, including the first trench isolation region 110 and the secondtrench isolation region 120) can prevent optical signal loss into thesemiconductor substrate 101.

Referring to the flow diagram of FIG. 12, also disclosed herein is amethod of forming, on a semiconductor-on-insulator (SOI) wafer, thesemiconductor structure 200, as described in detail above, comprising aphotodetector 230 on stacked trench isolation regions (i.e., the firsttrench isolation region 210 and the second trench isolation region 220).

Specifically, the method can comprise providing asemiconductor-on-insulator (SOI) wafer comprising a semiconductorsubstrate 201 having a top surface 205, an insulator layer 203 on thetop surface 205 of the semiconductor substrate 201 and a semiconductorlayer on the insulator layer 203 (1202). The semiconductor substrate 201can comprise, for example, a silicon substrate or other suitablesemiconductor substrate. The insulator layer 203 can comprise, forexample, a silicon dioxide layer or other suitable insulator layer. Thesemiconductor layer can comprise, for example, a silicon layer, asilicon germanium layer or other suitable semiconductor layer.

The method can further comprise forming a first trench isolation region210 in the semiconductor layer (1204, see FIG. 13). To form the firsttrench isolation region 210, a first trench 211 can be formed (e.g.,lithographically patterned and etched) through the semiconductor layer.This first trench 211 can subsequently be filled with one or more firstisolation materials 212 (e.g., silicon dioxide, silicon nitride, siliconoxynitride, and/or any other suitable isolation material). It should benoted that the first trench 211 can be patterned and etched such thatthe resulting first trench isolation region 210 has a first opening 215extending vertically there through and such that a portion 204 of thesemiconductor layer is within the first opening 215 laterally surroundedby the first trench isolation region 210. Thus, the shape of the firstopening 215 of the first trench isolation region 210 defines the shapeof this portion 204 of the semiconductor layer. This first opening 215can, for example, be an essentially rectangular-shaped first opening.

Optionally, during the process of forming the first trench isolationregion 210, which defines a portion 204 of the semiconductor layer,dielectric columns 217 can also be formed within the portion 204. Thatis, when the first trench 211 is lithographically patterned and etched,multiple vias can also be lithographically patterned and etch throughthe portion 204. When the first trench 211 is filled with the firstisolation material(s), the vias can simultaneously be filled in order toform the dielectric columns.

This method can further comprise forming a photodetector 230 on theportion 204 of the semiconductor layer within the first opening 215(1206, see FIG. 14). Specifically, a light-absorbing layer 231 can beformed (e.g., epitaxially deposited) over the portion 204 of thesemiconductor layer defined by the first trench isolation region 210 andfurther over the first trench isolation region 210. This light-absorbinglayer 231 can comprise a light absorbing material and, particularly, amaterial that absorbs light (i.e., optical signals or photons) withinpredetermined wavelength bands. For example, this light-absorbing layer231 can comprise a germanium layer (e.g., an epitaxial germanium layer)that absorbs light (i.e., optical signals or photons) in the infraredwavelength bands (i.e., wavelengths (λ) in the range of approximately1.2μ to 1.7μ).). The germanium layer can be in-situ doped duringepitaxial deposition (or subsequently doped) so as to have N-typeconductivity, P-type conductivity, or alternating regions of N-typeconductivity and P-type conductivity (e.g., to form a PN junction diodeor a multitude of PN junctions within the germanium layer). The dopingconcentrations can range from 1e17 atoms/cm³ to 1e21 atoms/cm³ withpreferable concentrations between 1e19 atoms/cm³ to 1e20 atoms/cm³. Thedoping profiles within the germanium layer can, for example, beconstructed such that the peak concentration is located at thehalf-height of the germanium layer. Furthermore, the doping profileswithin the germanium layer can be optimized to reduce dark current ofthe photodetector, while providing for high responsivity and highoperation speed. In another example, the light-absorbing layer 131 cancomprise a germanium-tin layer or any other suitable light-absorbinglayer. Such light-absorbing layers can similar be doped so as to haveN-type conductivity, P-type conductivity, or alternating regions ofN-type conductivity and P-type conductivity. Optionally, prior toformation of the light-absorbing layer 231, a buffer layer 232 can beformed (e.g., epitaxially deposited) and, following formation of thelight-absorbing layer 231, a dielectric cap layer 236 can be formed(e.g., deposited). The buffer layer 232 can comprise, for example, asilicon germanium layer (e.g., an epitaxially deposited silicongermanium layer) that facilitates subsequent epitaxial deposition of agermanium light-absorbing layer. The dielectric cap layer 236 cancomprise, for example, a silicon nitride cap layer or other suitabledielectric cap layer. Those skilled in the art will recognize that ifthe above-described optional buffer layer 232 and light-absorbing layer231 are formed by epitaxial deposition, such processes are typicallyfollowed by an anneal. In any case, the light-absorbing layer 231 and,if applicable, the buffer layer 232 and dielectric cap layer 236 cansubsequently be lithographically patterned and etched to form thephotodetector 230 such that the photodetector 230 is aligned above thefirst opening 215 and is slightly larger than the first opening 215 soas to extend laterally onto the first trench isolation region 210. Theresulting photodetector 230 can, for example, have essentially the sameshape as the first opening 215 (e.g., an essentially rectangular shape)with a first end 233, a second end 234 opposite the first end 233, andopposing sides.

It should be noted that the photodetector 230 can specifically be formedat process 1206 so that its height 239 will be less than the diameter259 of a core 251 of an optical fiber 250 to which it will be coupled inthe resulting semiconductor structure 200 (see FIG. 2A). Alternatively,the photodetector 230 can specifically be formed at process 1206 so thatits height 239 will be approximately equal to the diameter 259 of a core251 of an optical fiber 250 to which it will be coupled in the resultingsemiconductor structure 200 (see FIG. 4).

In any case, after forming the photodetector 230, a dielectric layer 241can be formed (e.g., conformally deposited) such that it covers thephotodetector 230 and further extends laterally onto the first trenchisolation region 210 (1208, see FIG. 15). This dielectric layer 241 cancomprise, for example, a silicon nitride layer, a silicon oxynitridelayer or any other suitable semiconductor layer.

Then, a second trench isolation region 220 can be formed in thesemiconductor substrate 201 below the insulator layer 203 and,specifically, in the semiconductor substrate 201 aligned below thephotodetector 230 and the first trench isolation region 210 (1210).Specifically, a plurality of second openings 216 can be formed (e.g.,lithographically patterned and etched) such that they extend verticallythrough the dielectric layer 241, the first trench isolation region 210and the insulator layer 203 and into the semiconductor substrate 201.These second openings 116 can further be formed (i.e., lithographicallypatterned and etched) such that they are positioned adjacent to multiplesides of the photodetector 230 and, particularly, adjacent to at leastthe first end 233 (i.e., the light-receiving end) and the opposing sides235 of the photodetector 230 (see FIGS. 16A-16B). Once the secondopenings 216 are formed, exposed surfaces of the semiconductor substrate201 in the lower sections of the second openings 216 can be etched untilthose lower sections are merged in order to form a second trench 221 inthe semiconductor substrate 201 below the insulator layer 203 and thissecond trench 221 and the second openings 216 can then be filled withone or more second isolation material(s) 222 so as to form the secondtrench isolation region 220 aligned below the photodetector 230 and thefirst trench isolation region 210 (see FIG. 17). The second isolationmaterial(s) 222 can comprise, for example, silicon dioxide, siliconnitride, silicon oxynitride and/or any other suitable isolationmaterial. The second isolation material(s) 222 can be the same as ordifferent from the first isolation material(s) 212.

The specifications for the above-described etch process used to form thesecond trench 221 should specifically be chosen so that thesemiconductor material of the semiconductor substrate 201 will beselectively etched over the materials used for the dielectric layer 241,the first trench isolation region 210 and the insulator layer 203. Forexample, if the semiconductor substrate 201 comprises silicon, if thefirst isolation material 212 of the first trench isolation region 210comprises silicon dioxide, if the insulator layer 203 comprises silicondioxide and if the dielectric layer 241 comprises silicon nitride, theetch process used to form the second trench 221 can comprise a wetchemical etch process, which uses an etchant, such astetramethylammonium hydroxide (TMAH), ammonium hydroxide (NH₄OH),ethylenediamine pyrocatechol (EDP), potassium hydroxide (KOH), or anyother suitable etchant capable of etching silicon over the variousdielectric and isolation materials. Those skilled in the art willrecognize that alternative etchants could be used depending upon thechemical differences between the semiconductor substrate 201, thedielectric layer 241, the first isolation material(s) 212 of the firsttrench isolation region 210, and the insulator layer 203. Those skilledin the art will recognize that such wet chemical etch processes alsotypically exhibit etch selectivity along crystal planes (e.g.,selectivity for silicon that is significantly higher in the [100]direction than in the [111] direction). As a result, the bottom surface225 of the second trench 221 may remain essentially parallel to the topsurface 205 of the semiconductor substrate 201, but the sidewalls 226may be angled, as opposed to perpendicular, relative to the top surface205 of the semiconductor substrate 201. It should be noted thatremaining sections of the dielectric layer 241, insulator layer 203 andfirst trench isolation region 210 (e.g., between the second openings216) as well as the adjacent substrate provide adequate support for thephotodetector 230 during etching of the second trench 221 and prior tofilling the second trench 221 with the second isolation material(s) 222.

Next, an antireflective (AR) spacer 260 can be formed such that it ispositioned laterally immediately adjacent to the first end 233 and,particularly, immediately adjacent to the light signal-receiving end ofthe photodetector 230 (1212, see FIG. 18). For example, a trench can belithographically patterned and etched through the dielectric layer 241and into the first trench isolation region 210 immediately adjacent tothe first end 233 of the photodetector 230. This trench can subsequentlybe filled with an antireflective (AR) material in order to form theantireflective (AR) spacer 260. The antireflective (AR) material cancomprise, for example, titanium nitride or any other suitableantireflective material. This antireflective (AR) spacer 260 should beformed at process 1212 so as to have a quarter-wave thickness ormultiple thereof. That is, the thickness of the antireflective (AR)spacer can be ¼ the wavelength of the optical signals, which areintended to be transmitted to and captured by the photodetector 230.

Following formation of the antireflective (AR) spacer 260, back end ofthe line (BEOL) processing can be performed in order to form contactsand other interconnects (e.g., wires and vias) in one or more additionaldielectric layers 242 (i.e., interlayer dielectrics such as, silicondioxide, silicon nitride, silicon oxynitride, borophosphosilicate glass(BPSG), etc.) above the dielectric layer 241 in order to electricallyconnect the photodetector 230 to one or more other on-chip devices(1214, see FIGS. 2A-2C). Additionally, an edge 290 of the semiconductorsubstrate 201 adjacent to the first end 233 of the photodetector 230 canbe prepared for receiving an off-chip optical fiber 250 so that opticalfiber 250 can be coupled to the photodetector 230 (1216, see FIGS.2A-2C). As mentioned above with regard to the semiconductor structure200, an optical fiber 250 can comprise a core 251 and cladding 252around this core 251. Both the core 251 and the cladding 252 cancomprise light-transmissive materials; however, the core material(s) canhave a refractive index that is higher than that of the claddingmaterial(s) so that light signals can be confined to and propagatedalong the core. To prepare the edge 290 of the semiconductor substrate201 for receiving an optical fiber 250, this edge 290 can be exposed(e.g., using a masked etch process) such that it extends laterallybeyond the first end 233 of the photodetector 230, the antireflective(AR) spacer 260, the first trench isolation region 210, the insulatorlayer 203 and the second trench isolation region 220. Then, a groove(e.g., a V-groove) for receiving the optical fiber 250 can be formed(e.g., lithographically patterned and etched) on the exposed edge 290such that it is aligned the photodetector 230.

After the edge 290 of the semiconductor substrate 201 is prepared forreceiving an optical fiber, as described above, one end of an opticalfiber 250 can be positioned within the groove adjacent to theantireflective (AR) spacer 260 such that it is in end-to-end alignmentwith the first end 233 of the photodetector 230 and, thereby such thatit is optically coupled to the photodetector 230 (1218, see FIGS.2A-2C). Once the optical fiber 250 is coupled to the photodetector 230in this manner, the optical fiber 250 can transmit optical signals tothe photodetector 230 and the photodetector 230 can convert those theoptical signals into electrical signals, which can, in turn, betransmitted to one or more other on-chip devices through the contactsand interconnects described above. During transmission of the opticalsignals from the optical fiber 250 to the photodetector 230, theisolation material that is below the photodetector 230 (e.g., in thestacked trench isolation regions, including the first trench isolationregion 210 and the second trench isolation region 220, as well as in theinsulator layer 203) can prevent optical signal loss into thesemiconductor substrate 201. Furthermore, in the semiconductor structure200 of FIGS. 2A-2C the optional dielectric columns 217 can be used tominimize optical signal loss into the portion 204 of the semiconductorlayer defined by the first trench isolation region 210.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should be understood that the terminology used herein is for thepurpose of describing the disclosed structures and methods and is notintended to be limiting. For example, as used herein, the singular forms“a”, “an” and “the” are intended to include the plural forms as well,unless the context clearly indicates otherwise. Additionally, as usedherein, the terms “comprises” “comprising”, “includes” and/or“including” specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. Furthermore, asused herein, terms such as “right”, “left”, “vertical”, “horizontal”,“top”, “bottom”, “upper”, “lower”, “under”, “below”, “underlying”,“over”, “overlying”, “parallel”, “perpendicular”, etc., are intended todescribe relative locations as they are oriented and illustrated in thedrawings (unless otherwise indicated) and terms such as “touching”,“on”, “in direct contact”, “abutting”, “directly adjacent to”, etc., areintended to indicate that at least one element physically contactsanother element (without other elements separating the describedelements). The corresponding structures, materials, acts, andequivalents of all means or step plus function elements in the claimsbelow are intended to include any structure, material, or act forperforming the function in combination with other claimed elements asspecifically claimed.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

Therefore, disclosed above are semiconductor structures and methods offorming the semiconductor structures. The semiconductor structures eachhave a photodetector that is optically and electrically isolated from asemiconductor substrate below by stacked trench isolation regions.Specifically, one semiconductor structure can comprise a first trenchisolation region in and at the top surface of a bulk semiconductorsubstrate and a second trench isolation region in the substrate belowthe first trench isolation region. A photodetector can be on the topsurface of the semiconductor substrate aligned above the first andsecond trench isolation regions. Another semiconductor structure cancomprise a semiconductor layer on an insulator layer and laterallysurrounded by a first trench isolation region. Additionally, a secondtrench isolation region can be in a semiconductor substrate below thefirst trench isolation region and insulator layer. A photodetector canbe on the semiconductor layer and can extend laterally onto the firsttrench isolation region. In each of these semiconductor structures, thefirst and second trench isolation regions (i.e., the stacked trenchisolations) can provide sufficient isolation below the photodetector toallow for direct coupling with an off-chip optical device (e.g., opticalfiber) with minimal optical signal loss through semiconductor substrate.

What is claimed is:
 1. A semiconductor structure comprising: asemiconductor substrate having a top surface; a first trench isolationregion in said semiconductor substrate at said top surface, said firsttrench isolation region having a first opening; a photodetector abovesaid first trench isolation region, the photodetector having at leastone layer extending laterally across said first opening so as to haveportions immediately adjacent to a top surface of the first trenchisolation region on opposite sides of said first opening; and, a secondtrench isolation region in said semiconductor substrate aligned belowsaid photodetector and said first opening in said first trench isolationregion, said first opening extending vertically between saidphotodetector and said second trench isolation region, said firstopening having an upper portion adjacent to said photodetector andfilled with semiconductor material and a lower portion adjacent to saidsecond trench isolation region and filled with isolation material. 2.The semiconductor structure of claim 1, said photodetector comprising alight-absorbing layer comprising any of a germanium layer and agermanium-tin layer.
 3. The semiconductor structure of claim 2, saidphotodetector further comprising a buffer layer between saidsemiconductor substrate and said light-absorbing layer.
 4. Thesemiconductor structure of claim 1, further comprising: a dielectriclayer on said photodetector and further extending laterally onto saidfirst trench isolation region; and, a plurality of second openingsextending vertically through said dielectric layer and said first trenchisolation region to said second trench isolation region, said secondopenings being positioned adjacent to multiple sides of saidphotodetector and being filled with said isolation material.
 5. Thesemiconductor structure of claim 1, said photodetector having a firstend and a second end opposite said first end and said semiconductorstructure further comprising: an antireflective spacer positionedlaterally adjacent to said first end; and, an optical fiber on saidsemiconductor substrate and in end-to-end alignment with said first endof said photodetector, said antireflective spacer being positionedlaterally between said optical fiber to said photodetector, said opticalfiber transmitting optical signals to said photodetector, and said firsttrench isolation region and said second trench isolation regionpreventing loss of said optical signals into said semiconductorsubstrate during said transmitting of said optical signals by saidoptical fiber.
 6. The semiconductor structure of claim 5, said opticalfiber comprising a core and cladding around said core, saidphotodetector having a height that is any one of less than a diameter ofsaid core and approximately equal to said diameter of said core.
 7. Asemiconductor structure comprising: a semiconductor substrate having atop surface; a first trench isolation region in said semiconductorsubstrate at said top surface; a photodetector above said first trenchisolation region; and, a second trench isolation region in saidsemiconductor substrate aligned below said photodetector and said firsttrench isolation region, said first trench isolation region having afirst opening that extends vertically between said photodetector andsaid second trench isolation region, said first opening having an upperportion adjacent to said photodetector and filled with semiconductormaterial and a lower portion adjacent to said second trench isolationregion and filled with isolation material.
 8. The semiconductorstructure of claim 7, said photodetector comprising a light-absorbinglayer comprising any of a germanium layer and a germanium-tin layer. 9.The semiconductor structure of claim 8, said photodetector furthercomprising a buffer layer between said semiconductor substrate and saidlight-absorbing layer.
 10. The semiconductor structure of claim 7,further comprising: a dielectric layer on said photodetector and furtherextending laterally onto said first trench isolation region; and, aplurality of second openings extending vertically through saiddielectric layer and said first trench isolation region to said secondtrench isolation region, said second openings being positioned adjacentto multiple sides of said photodetector and being filled with saidisolation material.
 11. The semiconductor structure of claim 7, saidphotodetector having a first end and a second end opposite said firstend and said semiconductor structure further comprising: anantireflective spacer positioned laterally adjacent to said first end;and, an optical fiber on said semiconductor substrate and in end-to-endalignment with said first end of said photodetector, said antireflectivespacer being positioned laterally between said optical fiber to saidphotodetector, said optical fiber transmitting optical signals to saidphotodetector, and said first trench isolation region and said secondtrench isolation region preventing loss of said optical signals intosaid semiconductor substrate during said transmitting of said opticalsignals by said optical fiber.
 12. The semiconductor structure of claim11, said optical fiber comprising a core and cladding around said core,said photodetector having a height that is any one of less than adiameter of said core and approximately equal to said diameter of saidcore.
 13. A semiconductor structure comprising: a semiconductorsubstrate having a top surface; a first trench isolation region in saidsemiconductor substrate at said top surface; a photodetector above saidfirst trench isolation region, said photodetector having a first end anda second end opposite said first end; a second trench isolation regionin said semiconductor substrate aligned below said photodetector andsaid first trench isolation region; an antireflective spacer positionedlaterally adjacent to said first end; and, an optical fiber on saidsemiconductor substrate and in end-to-end alignment with said first endof said photodetector, said antireflective spacer being positionedlaterally between said optical fiber to said photodetector, said opticalfiber transmitting optical signals to said photodetector, and said firsttrench isolation region and said second trench isolation regionpreventing loss of said optical signals into said semiconductorsubstrate during said transmitting of said optical signals by saidoptical fiber.
 14. The semiconductor structure of claim 13, saidphotodetector comprising a light-absorbing layer comprising any of agermanium layer and a germanium-tin layer.
 15. The semiconductorstructure of claim 14, said photodetector further comprising a bufferlayer between said semiconductor substrate and said light-absorbinglayer.
 16. The semiconductor structure of claim 13, said first trenchisolation region having a first opening that extends vertically betweensaid photodetector and said second trench isolation region, said firstopening being at least partially filled with isolation material.